In many applications it is necessary to have simultaneous transmission and reception of acoustic signals in the same frequency bands. This occurs frequently in applications involving hands-free communications where speech from a near-end talker must be acquired through a sound receiving transducer (such as a microphone) at the same time that speech from a far-end talker must be played back through a sound transmitting transducer (such as a loudspeaker).
A significant problem in the design of such systems is that the microphone intended only for near-end speech also picks up the far-end speech signal, played back using a loudspeaker. This acoustic and vibratory coupling problem manifests itself in two ways. First, if the far-end of the communications link also has some amount of coupling (acoustic or electrical), then the potential for instability or howling exists. Second, when the unintentionally acquired far-end signal is transmitted back to the far-end party, it is received as an audible echo. This echo, when delayed by propagation through the communications network, can be extremely annoying and in severe circumstances, can render the communications channel useless. The acoustic coupling problem is particularly acute when the loudspeaker and microphone are located in close proximity as in the case of a desktop handsfree telephone.
For ideal full-duplex operation in a loudspeaking telephone (i.e., simultaneous conversation in two directions), both parties in a telephone conversation must be permitted to speak and be heard simultaneously. This requires significant acoustic separation of the loudspeaker and microphone.
There are some general approaches that can reduce the coupling. The physical separation between loudspeaker and microphones should be as great as possible. Transducers can be mounted with an acoustically opaque structure inserted in the space between them. The loudspeaker should be oriented so that its maximal radiation (at high frequencies) is directed away from the microphones. If directional microphones are used, the nulls of the microphones can be directed toward the loudspeaker. Echo-cancellation techniques can be implemented in the electronics. A practical design usually employs more than one of these techniques to achieve full duplex operation.
Commonly, acoustic separation is increased in a simple way by increasing the distance between the loudspeaker and microphone. One such example of this is described in U.S. Pat. No. 4,378,468 issued Mar. 29, 1983 in the name of Daryl P. Braun (the Braun patent). The Braun patent describes an audio conference system that alleviates sound-coupled feedback by mounting the loudspeakers below the conference table (“preferably at floor level” according to the patent) while mounting the microphones above the table. While this approach does reduce acoustic coupling, its operation relies on the presence of a suitable table and it is not applicable to systems where the loudspeaker and microphone must, of necessity, be located in the same housing.
For a fixed and compact system size and when the loudspeaker and microphone are in the same housing, the effective distance between transducers can be increased by exploiting acoustic diffraction. Sound tends not to propagate around obstacles, corners or edges (hence, the utility of roadside noise barriers). The obstacles create “acoustical shadows”. The residual sound that does get around an obstacle does so through the mechanism of diffraction. The effects of diffraction can be predicted numerically using finite element or boundary element techniques.
For instance, if the transducers are mounted on opposite sides of an acoustically opaque object, sounds propagating from the loudspeaker to the microphone must propagate around the obstacle (assuming that no flanking transmission paths exist).
An example of this approach is described in U.S. Pat. No. 4,078,155 issued Mar. 7, 1978 in the names of R. Botros et al. (the Botros patent). The Botros patent describes an audio conference terminal housing consisting of a cylindrical section on top of an inverted conical section. The loudspeaker is mounted at the top of the cylinder while the microphone is mounted at the bottom of the inverted conical section to provide physical separation between the speaker and microphone.
Another approach involves the use of transducers with direction-dependent characteristics: loudspeakers, microphones or both. For example, acoustic coupling is reduced by mounting a directional microphone such that the direction of minimum sensitivity coincides with the direction of the loudspeaker.
Many examples of this design approach can be found. U.S. Pat. No. 3,992,586 issued Nov. 16, 1976 in the name of Christopher Jaffe generates a directional loudspeaker pattern by driving two omni-directional loudspeakers out of phase. By positioning the microphone in the acoustic null zone of the resulting dipole, acoustic coupling is reduced. U.S. Pat. No. 4,237,339 issued Dec. 2, 1980 in the names of Bunting et al. describes a boom on which directional microphones and a loudspeaker are rigidly mounted such the microphone nulls are directed towards the loudspeaker.
A similar approach has also been used to design compact speakerphone housings as described in U.S. Pat. No. 5,121,426 issued Jun. 9, 1992 in the names of John Baumhauer et al., U.S. Pat. No. 5,896,461 issued Apr. 20, 1999 in the names of Philip Faraci et al., and U.S. Pat. No. 6,016,346 issued Jan. 18, 2000 in the names of Stephen Rittmueller et al.
The current approaches described above have limitations. The acoustic separation achieved simply by increasing distance is not applicable to small devices. Similarly, acoustic diffraction losses are significant only when the diffracting object is an appreciable fraction of a wavelength in dimension. For the acoustic wavelengths at speech signal frequencies, this implies rather large devices. Finally, approaches involving directional transducers place restrictions on the placement of these transducers which is unacceptable in some instances.
Therefore, a method and apparatus for decreasing the acoustic coupling between a sound receiving transducer (such as a microphone) and a sound transmitting transducer (such as a loudspeaker), particularly when such transducers are mounted in close proximity or in the same physical housing as occurs in the design of hands-free telephones, is needed.